1. Field of the Invention
Generally, the present disclosure relates to metrology in the manufacturing of microstructures, such as integrated circuits, and, more particularly, to the measurement of the surface characteristics of microstructure features by means of scanning probe microscopy (SPM) for determining surface and near-surface characteristics with nanometer resolution.
2. Description of the Related Art
In manufacturing microstructures, such as integrated circuits, micromechanical devices, opto-electronic components and the like, device features, such as circuit elements, are typically formed on an appropriate substrate by patterning the surface portions of one or more material layers previously formed on the substrate. Since the dimensions, i.e., the length, width and height, of individual features are steadily decreasing to enhance performance and improve cost-effectiveness, these dimensions have to be maintained within tightly set tolerances in order to guarantee the required functionality of the complete device. Usually a large number of process steps have to be carried out for completing a microstructure, and thus the dimensions of the features during the various manufacturing stages have to be thoroughly monitored to maintain process control and to avoid further cost-intensive process steps owing to process tools that fail to meet the specifications in an early manufacturing stage. For example, in highly sophisticated CMOS devices, the gate electrode, which may be considered as a polysilicon line formed on a thin gate insulation layer, is an extremely critical feature of a field effect transistor and significantly influences the characteristics thereof. Consequently, the size and shape of the gate electrode has to be precisely controlled to provide the required transistor properties. Thus, great efforts are being made to steadily monitor the dimensions of the gate electrode. However, for highly scaled microstructures, the surface characteristics of the materials used also increasingly become important aspects for enhancing performance and reliability of the devices and/or for reducing yield loss and the like. For example, the detection of grain sizes, micro-cracks, adhesion characteristics and elastic properties may be important for the performance and reliability of the devices, especially when highly scaled microstructure devices are considered. By way of example, in metallization layers of advanced integrated circuits, the adhesion characteristics may significantly affect the mechanical strength during the manufacturing process and may also determine the reliability of the finished device while also the crystallinity of the metal influences the current drive capability and thus the performance of the device. Moreover, the formation of well-understood interfaces between two different materials is an important aspect and may therefore require reliable measurement data of surface characteristics.
Consequently, there was a growing need for techniques which may be able to characterize materials and in particular their engineered surfaces with a spatial resolution that is appropriate for highly scaled microstructures, such as integrated circuits. Frequently used and very powerful tools in this respect are the atomic force microscope (AFM) and the scanning tunnel microscope (STM) which allow the characterization of nano-structured materials by scanning an appropriate tip of a cantilever across a sample surface. During the scan operation, typically the charge cloud of the tip interacts with respective charge clouds of the sample surface, wherein the corresponding interaction, i.e., the minimal displacements of the tip or the tunnel current, are recorded to obtain information on the surface structure with a nanometer resolution. Thus, scanning probe microscopy provides a three-dimensional image of the surface topography, which may provide precious information with respect to the surface structure of the sample. When imaging a sample surface with high spatial resolution, respective surface portions within an area of a few micrometers or smaller may appear almost atomically flat so that the corresponding image obtained by means of the scanning probe microscope may include less contrast and thus may not allow efficient extraction of detailed information on surface characteristics at this high resolution, although on a broader scale a significant sample topography may be present.
Therefore, a plurality of probing techniques have been developed, in which image contrast and thus extraction of details of small areas of interest may be efficiently enhanced by initiating an interaction of the probe with the sample surface, by, for instance, exciting the sample surface with sound waves, wherein ultrasonic sound with frequencies up to several MHz may be used. In this case, the tip of the probe may act as a means for interacting with the surface and also as a detector for recording the response of the surface to the excitation. However, many other mechanisms have been developed to interact with the sample surface based on the principle of using the highly local influence of the tip of a scanning probe on the sample surface. For instance, an electric field may be created or modified in a highly localized manner, which may be advantageously used in strain measurement techniques.
Recently, strain-inducing techniques have been established in an attempt to further enhance performance of silicon-based circuit elements, since strained silicon exhibits a modified charge carrier mobility compared to non-strained silicon. For this reason, various techniques have been proposed for creating a specific type and magnitude of strain in the channel regions of silicon-based field effect transistors to obtain enhanced drive current capability and thus switching speed. The strain in the channel regions and/or other transistor areas may have to be specifically applied for the different transistor types, such as N-channel transistors and P-channel transistors, since the type of strain that enhances performance of one type of transistor may result in performance degradation in another type of transistor. Thus, sophisticated strategies may be required to create the desired type and magnitude of strain in a highly localized manner, thereby also calling for advanced measuring techniques that may allow the determination of strain characteristics in a spatially resolved manner at high precision and reliability. In this respect, Raman spectroscopy has been proven to be a viable candidate, since internal strain may be determined on the basis of the shift of spectral lines. It has been demonstrated that the sensitivity of this metrology technique may be greatly enhanced by locally modifying the electric field at the surface portion under consideration by positioning a probe having a tip that may interact with the surface to achieve the desired modification of the local electric field.
In other cases, the temperature distribution may have to be determined with high spatial resolution, which may require a thermal and electrical probe that, however, should not significantly affect the temperature of the surface portion to be measured, thereby requiring a thorough balancing of thermal conductivity characteristics and low overall mass at appropriate mechanical stability of the nano-probe.
Thus, since the introduction of STM and AFM techniques, a multitude of nano-probes have been developed on the basis of modifications of the initial concept, thereby resulting in a plurality of new analytical techniques. For each of these analytical tools used for the determination of various surface-specific characteristics, a specific nano-probe has to be manufactured in such a way that a tip portion that is connected to an appropriate cantilever provides the possibility for an interaction with the sample surface to be examined on the basis of nanometer dimensions. For this purpose, typically a tip portion with a nano-sized curvature of the apex may be formed by wet or dry etching of an appropriate structural material, such as silicon, by, for instance, taking advantage of the different etch characteristics of different crystallographic orientations for certain etch chemistries. Thus, depending on the basic lithography and etch techniques used, the tip portion of the nano-probe may be manufactured in an appropriate shape as required for the mechanical probing of the surface under consideration. That is, the nano-probes formed on the basis of well-established lithography and etch techniques may be used as nano-mechanical “interfaces” interacting with the surface portion under consideration, thereby meeting the requirements for a wide class of applications, in which the mechanical interaction represents the main aspect of the analytical technique of interest. In other applications, however, additional requirements beyond the pure mechanical interaction have to be taken into consideration. For electrical nano-probing for measurement techniques requiring electrical interaction with the sample surface, the tip portion has to be conductive and also requires appropriate connections with the cantilever portion for signal propagation.
In other aspects involving thermal characteristics of the surface under consideration, as previously mentioned, even more challenging requirements of such a nano-probe have to be taken into consideration. For example, a corresponding probe requires, in addition to the nano-size tip portion to be in contact with the sample, a transducer portion enabling the measurement of the temperature has to be provided at the apex of the tip portion, while, at the same time, the thermal mass has to be as small as possible to reduce the overall response time and also maintain a thermal influence on the surface portion to be measured at a low level. Moreover, thermal conditions occurring in the vicinity of the tip portion should not significantly affect the measurement results obtained by the interaction of the tip portion with the sample surface. For example, heating of the cantilever portion by the laser used to detect the minute deflections of the cantilever portion caused by the interaction of the tip portion with the sample surface should not significantly modify the actual thermal measurement results.
Thus, a plurality of promising analytical techniques are available on the basis of scanning probe microscopy techniques, which may be used for estimating materials and manufacturing techniques applied for the manufacturing of microstructural devices, such as advanced integrated circuits, wherein, however, the fabrication of appropriate nano-probes on the basis of conventional strategies may reduce the overall efficiency of these promising analytical tools.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.